Forming a beam from a subscriber module of a fixed wireless access communication system

ABSTRACT

A subscriber module of a fixed wireless access communication system comprises an offset Gregorian antenna arrangement, an array of antenna elements arranged as a feed, a beamforming network and a processor. The processor is configured to provide, to the beamformer, a pre-determined plurality of antenna weight vectors configured to form a plurality of beams, the orientations of the plurality of beams being arranged in a grid comprising a plurality of rows, each of the pre-determined plurality of antenna weight vectors being configured to form a respective beam from the primary reflector dish of the Gregorian antenna arrangement by forming a respective feed beam from the array of antenna elements. The relationship between the azimuth and elevation direction of each feed beam and the azimuth and elevation direction of the respective beam from the primary reflector dish is a non-linear function of azimuth and elevation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from UK Patent Application No. GB2019618.4, filed on Dec. 11, 2020, the entirety of which is hereby fullyincorporated by reference.

TECHNICAL FIELD

The present invention relates to a subscriber module of a fixed wirelessaccess communication system, the subscriber module comprising an offsetGregorian antenna arrangement comprising a primary reflector dish and asecondary reflector and an array of antenna elements arranged as a feedfor the secondary reflector, and to a method of forming a beam from thesubscriber module.

BACKGROUND

There is a growing market for wireless systems operating at increasinghigh frequencies as demand for increased bandwidth continues and as thecost of radio frequency electronic devices falls. In particular forfixed wireless access systems, there is a requirement for radio stationsto have a high antenna gain to provide sufficient system gain toestablish communication over long distances, which may be of the orderof a kilometre or further, at higher frequencies, which may by up to 60GHz or even higher. To provide high gain antenna beams, an array ofantenna elements may be conventionally provided in which the amplitudeand/or phase of each antenna element is controlled by a beamformer toproduce beams. However, the gain of a beam provided by the array ofantenna elements may be limited by the number of elements in the array.It may be required to produce beams having a greater gain than may beprovided by a given array.

SUMMARY

In accordance with a first aspect of the invention there is provided asubscriber module of a fixed wireless access communication system, thesubscriber module comprising:

an offset Gregorian antenna arrangement comprising a primary reflectordish and a secondary reflector;

an array of antenna elements arranged as a feed for the secondaryreflector, the array of antenna elements and the secondary reflectorbeing offset in a vertical axis with respect to a centre of the primaryreflector dish;

a beamforming network, the beamforming network being configured to forma beam using an antenna weight vector; and

a processor configured to provide an antenna weight vector selected froma pre-determined plurality of antenna weight vectors to the beamformer,

wherein the processor is configured to provide the pre-determinedplurality of antenna weight vectors configured to form a plurality ofbeams, the orientations of the plurality of beams being arranged in agrid comprising a plurality of rows, each of the pre-determinedplurality of antenna weight vectors being configured to form arespective beam from the primary reflector dish of the Gregorian antennaarrangement by forming a respective feed beam from the array of antennaelements,

wherein the relationship between the azimuth and elevation direction ofeach feed beam and the azimuth and elevation direction of the respectivebeam from the primary reflector dish is a non-linear function of azimuthand elevation.

Providing the offset Gregorian antenna arrangement provides a convenientmethod of increasing the gain of beams provided by the array of antennaelements. Offsetting the secondary reflector in a vertical axis withrespect to a centre of the primary reflector dish allows beams to beformed over a broad range of azimuth angles without obstruction of thebeam by the secondary reflector or the array. Providing a predeterminedplurality of weight vectors is computationally efficient, by allowingthe calculation of the weight vectors to be performed in advance offorming the beams. Arranging the orientations of the plurality of beamsin a grid comprising a plurality of rows allows series of beams to beformed at different azimuth angles and at the same elevation, whichallows a convenient method of forming trial beams, for example toestablish initial communication between the subscriber module and anaccess point of the wireless communication system. This also allows aconvenient method for re-selection of beams to track movement of asubscriber module due to wind loading, for example if the subscribermodule is mounted on a pole above a subscriber's premises. Providing thepredetermined plurality of weight vectors such that the relationshipbetween the azimuth and elevation direction of each feed beam and theazimuth and elevation direction of the respective beam from the primaryreflector dish as a non-linear function of azimuth and elevation allowsthe plurality of beams formed from the subscriber module to be arrangedas a series of straight rows in a grid, by arranging the feed beams fromthe array of antenna elements as a distorted grid. This allows theprocessor to apply a simple algorithm to steer beams by selection ofbeams in a straight row.

In an example, the pre-determined plurality of antenna weight vectors isconfigured to form the plurality of feed beams such that theorientations of the plurality of beams is arranged in a distorted gridcomprising a plurality of curved rows, each curved row providing amonotonic change in azimuth angle along the curved row, and anon-monotonic change in elevation angle along the curved row. Eachcurved row may have an offset in elevation angle between the centre ofthe curved row and either end of the curved row. The offset in elevationangle for a curved row may be equal to the angular spacing in elevation,at the centre of the curved row, between the curved row and an adjacentcurved row +/−50%. This provides the plurality of beams as a gridcomprising the plurality of rows which have a constant elevation to agood approximation. Typically, each curved row may have a greaterelevation angle at the centre of the curved row than at either end ofthe curved row. In an example, each curved row has an approximatelyparabolic dependence of elevation angle on azimuth angle, within +/−50%of a true parabola.

In an example, the array of antenna elements has 8 element columns and 8element rows with a spacing between antenna elements in each element rowand in each element column of substantially half a wavelength at anoperating frequency of the wireless communication system. This allowscommercially available antenna arrays to be used, which may, forexample, be arranged to form approximately 100 beams, but which may notprovide sufficient gain without the offset Gregorian antennaarrangement.

In accordance with a second aspect of the invention, there is provided amethod of forming a beam from a subscriber module of a fixed wirelessaccess communication system, the subscriber module having an offsetGregorian antenna system comprising a primary reflector dish, asecondary reflector, an array of antenna elements and a beamformingnetwork, the beamforming network being configured to form a beam usingan antenna weight vector selected from a pre-determined plurality ofantenna weight vectors, wherein the array of antenna elements isarranged to feed the secondary reflector to form the beam from theprimary reflector dish, the method comprising:

providing the pre-determined plurality of antenna weight vectorsconfigured to form a plurality of beams, the orientations of theplurality of beams being arranged in a grid comprising a plurality ofrows, each of the pre-determined plurality of antenna weight vectorsbeing configured to form a respective beam from the primary reflectordish of the Gregorian antenna arrangement by forming a respective feedbeam from the array of antenna elements,

wherein the array of antenna elements and the secondary reflector areoffset in a vertical axis with respect to a centre of the primaryreflector dish, and the relationship between the azimuth and elevationdirection of each feed beam and the azimuth and elevation direction ofthe respective beam from the primary reflector dish is a non-linearfunction of azimuth and elevation.

Further features and advantages of the invention will become apparentfrom the following description of examples of the invention, which ismade with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the present invention may be more readily understood,examples of the invention will now be described, with reference to theaccompanying drawings, in which:

FIG. 1 is a schematic diagram showing a wireless communication systemhaving an access point and subscriber modules each having an array ofantenna elements;

FIG. 2 shows a first wireless station configured to form a plurality ofbeams;

FIG. 3 shows a first and second wireless station each configured to forma plurality of beams;

FIG. 4 a shows a wireless station having an array of antenna elementsand a beamformer;

FIG. 4 b shoes the array of antenna elements of FIG. 4 b;

FIG. 5 shows a rectangular grid of pre-configured antenna beams;

FIG. 6 shows an antenna plot of the rectangular grid of antenna beams ofFIG. 5 ;

FIG. 7 shows a detail of FIG. 6 ;

FIG. 8 shows a grid of pre-configured beams in a triangular arrangement;

FIG. 9 shows an antenna gain plot of the triangular grid of FIG. 8 ;

FIG. 10 shows a detail of FIG. 9 ;

FIG. 11 shows an antenna gain plot of the triangular grid of a subset ofbeams of FIG. 8 , formed by a sub-set of the pre-determined plurality ofantenna weight vectors;

FIG. 12 shows a grid of pre-configured beams in a triangular arrangementhaving spacing on a row in proportion to beamwidth;

FIG. 13 shows an antenna gain plot of the grid of FIG. 12 ;

FIG. 14 is a schematic diagram showing the principle of operation of anoffset Gregorian antenna arrangement with a planar array of antennaelements as a feed;

FIG. 15 shows a plurality of feed beams formed to feed a secondaryreflector of an offset Gregorian antenna arrangement;

FIG. 16 shows a schematic diagram of a cross-section of the offsetGregorian antenna arrangement;

FIGS. 17 a and 17 b are schematic diagrams showing the shape the primaryreflector dish and the secondary reflector in a cross-section in avertical and horizontal cross-section respectively;

FIG. 18 shows an oblique perspective view of a first wireless stationhaving the offset Gregorian antenna arrangement;

FIG. 19 is a plan view of the first wireless station, viewed from thedirection of a radiofrequency main beam which the offset Gregorianantenna arrangement is configured to form;

FIG. 20 shows a grid of a plurality of pre-configured beams formed fromthe primary reflector of an offset Gregorian antenna arrangement;

FIG. 21 is an antenna plot of the grid of FIG. 20 ;

FIG. 22 shows a plurality of feed beams from the array of antennaelements as a feed to the secondary reflector of a Gregorian antennasystem; and

FIG. 23 is a flow diagram of a method according to an example.

DETAILED DESCRIPTION

Examples of the invention are described in the context of a terrestrialfixed wireless access wireless communication system operating in theband of 57-66 GHz operating according to IEEE 802.11ay. In the describedexamples, the wireless communication system is a time division duplexwireless system. However, it will be understood that embodiments of theinvention may relate to other applications, and to other frequencybands.

FIG. 1 shows a wireless communication system having an access point 1and subscriber modules 2 a, 2 b, 2 c, 3 a, 3 b, in a schematic planview. The access point 1 as shown covers two sectors, having two planararrays of antenna elements 4, 5 arranged to cover a first sector, andtwo further antenna arrays 6, 7 arranged to cover a second sector. Eachof the arrays is arranged to form beams within approximately +/−40degrees in azimuth and +/−20 degrees in elevation of the bore sightdirection of the array, that is to say perpendicular to the plane of thearray. The two arrays covering a sector are arranged to have boresightdirections which are different by approximately 80 degrees, so thatbeams may be formed in a continuous angular sector of approximately 160degrees using the two arrays. Each element of the array of antennaelements is connected to a beamformer, which may be in the form of acommercially available beamforming radiofrequency integrated circuitarranged to apply a selected weighting vector comprising a respectivetransmission phase for each element of the array. For example, the arrayof antenna elements may be an 8×8 array of patch antenna elements spacedapart by approximately a half wavelength. The beamformer may betypically arranged to form a beam selected from a number ofpre-configured beams, in an example 120 pre-configured beams. Thepre-configured beams may be distributed over an angular sector ofapproximately +/−40 degrees in azimuth and +/−20 degrees in elevation.

In the fixed wireless access wireless communication system shown in FIG.1 , the access point 1 is typically located on a tower, and thesubscriber modules may be a mix of high gain 3 a, 3 b and lower gain 2a, 2 b, 2 c subscriber modules, typically fixed to poles mounted atsubscribers' premises, which may be commercial or private residentialpremises, for example. The lower gain subscriber module 2 a, 2 b, 2 chave an antenna arrangement comprising a similar array of antennaelements to that used at the access point, and may be installedrelatively close to the access point, typically within a few hundredmetres. The high gain subscriber modules 3 a, 3 b have an antennaarrangement comprising a similar or the same array of antenna elementsto that used at the access point, but the array is used as a feed for anoffset Gregorian antenna arrangement, which gives an improved antennagain and a narrower antenna beam,

For the lower gain subscriber modules 2 a, 2 b, 2 c, the array ofantenna elements may be the same 8×8 array of patch antenna elementsused at the access point, and the beamformer may also be arranged toform a beam selected from 120 pre-configured beams distributed over anangular sector of approximately +/−40 degrees in azimuth and +/−20degrees in elevation in one example. To establish communication on firstinstallation, the lower gain subscriber module 2 a, 2 b, 2 c is alignedroughly in the direction of the access point, and the best beam for usecan be selected by a sweep of possible beams at the subscriber modulealso sweeping possible beams at the access point, which my be anexhaustive search of each combination of beams, so that a best beam atthe subscriber module and a best beam at the access point can beselected.

The higher gain subscriber modules 3 a, 3 b may be installed furtherfrom the access point, for example at distances of 1 km or more. Thehigher gain antenna arrangement may overcome the greater signal loss duethe greater propagation distance and the effects of signal loss due tooxygen absorption and rain in the approximately 60 GHz band.

The high gain subscriber modules 3 a, 3 b typically use the same arrayof antenna elements and the same beamforming arrangement as used at theaccess point 1 and the lower gain subscriber modules 2 a, 2 b, 2 c, as afeed for the offset Gregorian antenna system. The beam produced by thearray of antenna elements is reflected by the secondary reflector of theoffset Gregorian antenna system onto the primary reflector dish, toproduce a narrower beam from the primary reflector dish than the beamproduced by the array. For example, the beam produced by the array maybe approximately +/−8 degrees between 3 dB points and the beamtransmitted or received by the primary reflector dish may beapproximately 0.7 degrees between 3 dB points. This reduced beamwidthgives an improvement in gain, which may provide approximately a 22 dBincrease in gain in comparison with the gain of the antenna array alone.The overall gain of the antenna arrangement of the high gain subscribermodule may be approximately 44 dBi (deciBels compared to isotropic) forthis arrangement. The high gain antenna arrangement results in areduction in the angular sector over which a beam may be formed. In theabove example, the pre-configured beams may be distributed over anangular sector of approximately +/−2 degrees in azimuth and +/−1 degreein elevation from the primary reflector dish. The same technique ofusing a scan of the beams at the access point and the subscriber moduleis used to find a best beam, as for the lower gain subscriber modules.As a result of the narrower beams, and the smaller angular sector overwhich the beams may be steered, an optical sight attached to the highgain subscriber module is typically used to first of all install thesubscriber module in an orientation in which the angular sector overwhich the beams may be steered includes the direction of the accesspoint.

FIG. 2 shows a first wireless station 8, which may be a subscribermodule or an access point, configured to form a plurality of beams 10using an array of antenna elements 13, and a second wireless station 9,typically the other of the access point and the subscriber module,configured to form a fixed beam 11. In this case, only the firstwireless station 8 performs the sweep to first beam.

FIG. 3 shows a first 8 and second 9 wireless station each configured toform a plurality of beams, 10, 12. The first wireless station may be asubscriber module and the second wireless station may be an accesspoint. In another configuration, more than one access point may be usedto form a meshed communication system. In this case, the first 8 andsecond 9 wireless stations may both be access points, and each willselect a best beam for use by a search process as already described.

FIG. 4 a shows a wireless station, which may be the first and/or thesecond wireless station. The wireless station has an array of antennaelements 22 and a beamformer 23. FIG. 4 b shows that the array ofantenna elements 22 comprises a two dimensional planar array ofelements. The antenna elements may be conventional patch antennaelements formed by conductive metal film carried on a non-conductivesubstrate such as a ceramic tile or conventional printed circuit boardmaterial. Two, or more, planar arrays may be arranged with differentboresight directions in azimuth in order to allow beams to be formedover a larger angular sector. The beamformer applies a weighting to thesignal transmitted and/or received by each antenna element. Typically,the weighting is a transmission phase value. The transmission phasevalue is typically quantised, for example to allow switching by 90degree steps. The application of the transmission phase value to thesignal may be implemented by use of a switchable transmission delay. Acombiner/splitter tree connects each element of the beamformer via afrequency converter stage to a radio modulator/demodulator 25, toconvert received signals and/or signals for transmission to and fromdigital format. A processor and controller 26 controls the beamselection and acquisition stage. Pre-configured beams are stored inmemory 24, for application to the beamformer 23 under control of theprocessor 26. The processor may be implemented using conventionaldigital techniques, and may be implemented in software, firmware orcloud-based processing. The formation of beams using phase weightsapplied to signals received by and/or transmitted by antenna elements iswell known in the art. For example, a beam with a conventional sincfunction beam shape may be formed by applying weights the antennaelements with uniform gain and with an appropriate phase slope acrossthe array in each dimension to steer the beam in the desired direction.The antenna array and beamformer may be commercially available items,for example the Samsung SWL-QD46 module.

FIG. 5 shows that the pre-determined plurality of antenna weight vectorsmay be configured to form a plurality of beams 35 a, 35 b, etc, theorientations of the plurality of beams being arranged in a rectangulargrid. The reference numerals are shown as examples on only a few beamsfor clarity, but each of the beams shown is one of the plurality beams.This approach allows a search of beams for acquisition using aconventional search in a two-dimensional plane arranged as rows andcolumns as shown. By this approach, may be easily searched byincrementation of an index in each orthogonal dimension, typically inazimuth and elevation.

FIG. 6 shows a plot of antenna gain for the beam arrangement of FIG. 5 ,showing the maximum gain that can be achieved at each point in the gridby selection of the best beam. FIG. 7 shows part of FIG. 6 in moredetail. It can be seen that, in the troughs 36 between peaks of beams35, the gain is approximately 2.4 dB below that of the peaks of thebeams. The contours are in steps of 0.2 dB.

By contrast, FIG. 8 shows that the pre-determined plurality of antennaweight vectors may be configured to form a plurality of beams 27 a, 27b, etc., the orientations of the plurality of beams being arranged in anarrangement of equilateral triangles. The reference numerals are shownas examples on only a few beams for clarity, but each of the beams shownis one of the plurality beams, that is to say each of the beams numbered1-120. The numbers in circles 28 a, 28 b, etc., are beams in the firstsubset of the plurality of beams used for an initial search and thenumbers in squares 39 a, 39 b, etc., are beams in the second subset ofthe plurality of beams used for a refined search once a first beam, inthis example beam number 66, that can allow communication the secondstation has been determined. As may be seen in FIG. 8 , the beams ofeach row are spaced in angular position in the row on a first axis 32,such that at least one beam in a respective row is positioned mid-way onthe first axis between the positions on the first axis of two beams onan adjacent row. For example, beam number 22 in row 34 b is mid-way onthe azimuth axis 32 between beam numbers 9 and 10 in row 34 a.

FIG. 9 shows a plot of antenna gain for the beam arrangement of FIG. 8 ,showing the maximum gain that can be achieved at each point in thearrangement by selection of the best beam. FIG. 10 shows part of FIG. 9in more detail. The contours are in steps of 0.2 dB. It can be seenthat, in the troughs 37 between peaks of beams 27, the gain isapproximately 1.8 dB below that of the peaks of the beams. Thisarrangement gives an improvement over the rectangular arrangement ofFIG. 5 of about 0.6 dB in the minimum gain available. This improvementprovides addition link margin to allow communication and acquisition inadverse atmospheric conditions.

To establish wireless communication between a first station and a secondstation in a wireless communication system, the following method may beused. The first station has an antenna comprising an array of antennaelements and a beamforming network, the beamforming network beingconfigured to form a beam using an antenna weight vector selected from apre-determined plurality of antenna weight vector. The pre-determinedplurality of antenna weight vectors may be referred to as a codebook.

The pre-determined plurality of antenna weight vectors are configured toform a plurality of beams. The position of each of the plurality ofbeams is shown overlaid as illustrated in FIG. 8 . The orientations ofthe beams are arranged in a grid comprising a plurality of rows 34 a, 34b, etc., the beams of each row being spaced in angular position in therow by a first angular separation 29 on a first axis 32, in this exampleazimuth. The angular positions of the beams of each row is offset 30 onthe first axis 32 by half of the first angular separation 29 withrespect to the angular positions of beams in an adjacent row. Forexample, it can be seen that each beam 27 a, 27 a, etc., in row 34 a isoffset by half the separation between beams 29 with respect to theangular positions of beams in an adjacent row 34 b. The beam numbers1-120 shown in FIG. 8 are arbitrary.

As shown in FIG. 8 . each row is separated from an adjacent row by thefirst angular separation 29 multiplied by cosine 30 degrees on a secondaxis, perpendicular to the first axis 32, so that each beam of theplurality of beams is arranged as an equilateral triangle with twoadjacent beams. For example, row 34 a is separated from row 34 b by thefirst angular separation 29 multiplied by cosine 30 degrees. In practicethe positions of the beams will be subject to errors due to the accuracyof the beamforming weights, taking into account the effects ofquantisation.

For an initial acquisition search, a first sub-set of the pre-determinedplurality of antenna weight vectors is selected, as shown as circledbeams 28 a, 28 b, etc. In this case 30 beams are selected for thesub-set, being 1 in 4 beams.

FIG. 11 shows an antenna gain plot of the triangular grid of the subsetof beams 28 c, 28 d, etc., formed by a sub-set of the pre-determinedplurality of antenna weight vectors. The sub-set, which is referred toas the first sub-set, is selected to form selected beams on alternaterows of the grid, the selected beams of each alternate row being spacedin angular position on the first axis by twice the first angularseparation, and the angular position of the selected beams of eachalternate row being offset on the first axis by the first angularseparation. This arranges each selected beam in the subset as anequilateral triangle with two adjacent selected beams. This arrangementprovides a minimum gain between peaks of beams in the sub-set ofapproximately −5.5 dB. This gives an improvement in link margin forinitial acquisition compared with the minimum gain between peaks for arectangular grid of beams because of the tighter packing of thetriangular arrangement.

In an alternative another proportion of the beams may be selected forthe sub-set other than 1 in 4, for example 1 in 9 beams. In each case atriangular arrangement shows an advantage.

In the acquisition process, a succession of beams is formed in a firsttime sequence at the first station using the first sub-set of thepre-determined plurality of antenna weight vectors to send firstmessages.

Dependent on the receipt of a first message at the second station usinga first beam at the first station, a refined search is carried out usinga further succession of beams is formed at the first station using asecond sub-set of the pre-determined plurality of antenna weight vectorsselected to form beams adjacent to the first beam. As shown in FIG. 8 ,marked by squares, the second sub-set is selected to form at least aring of six beams surrounding the respective first and second beams ifthe first or second beam is not at an edge of the grid. The secondsub-set of beams is shown in FIG. 8 by the beams marked by squares, 39a, 39 b, etc., which surround the first beam 38 which was selected inthe initial acquisition process. This provides an efficient process forselecting a best beam for use after establishing initial communication.

As already mentioned, the array of antenna elements of the first stationmay be arranged to feed a secondary reflector of an offset Gregorianantenna arrangement, to increase the antenna gain. This may bebeneficial for a subscriber module, an access point, or for a wirelessstation arranged in a mesh arrangement where more gain is required.

FIG. 12 shows an alternative arrangement of the grid of pre-configuredbeams in a triangular arrangement having spacing on a row in proportionto beamwidth. It can be seen that beams at the centre of a row 27 f, 27g have a closer angular spacing than beams towards the end of a row 27h, 27 i. The spacing is constant in terms of a proportion of the 3 dBbeamwidth of a beam. This may provide a means of reducing the number ofbeams which need to be searched in an acquisition process. The reductionof gain between the areas between beams may be kept approximatelyconstant across the grid of beams by this approach.

FIG. 13 shows an antenna gain plot of the grid of FIG. 12 in schematicform.

FIG. 14 is a schematic diagram showing the principle of operation of anoffset Gregorian antenna arrangement, having a primary reflector dish 43and a secondary reflector 42. An array of antenna elements 41 is used tofeed the secondary reflector 42 with radiofrequency radiation formedinto a first beam having a first beamwidth. The amplitude and/or phaseof the signals fed to/received from respective elements of the array arearranged to have appropriate values to form a beam of intended directionand beamwidth. The amplitude and/or phase of the signals fed to/receivedfrom respective elements is typically controlled by a beamformerimplemented by a radiofrequency integrated circuit. The effect of thecombination of the primary reflector dish 43 and the secondary reflector42 is to increase the gain of the first beam, producing a second beam ofreduced beamwidth. For example, the first beam may have a beamwidth,measured as being the angular distance between points of the radiationbeam that have a gain 3 dB lower than the gain in the centre of thebeam, of approximately 8 degrees, and the second beam may have abeamwidth of approximately 0.5 degrees.

FIG. 15 shows a plurality of feed beams 45 formed from the array ofantenna elements 41 to feed the secondary reflector 42 of the offsetGregorian antenna arrangement, to produce a plurality of beams 46 fromthe primary reflector dish 43. It can be seen that a given deviation ofa feed beam from a direction perpendicular to the array will result in asmaller deviation in the beam of from the primary reflector dish 43. Asa result, the angular sector in which beams 46 from the primaryreflector dish may be formed is narrower than the angular sector inwhich beams 45 from the primary reflector dish may be formed. Each feedbeam corresponding to a respective one of the plurality of beams fromthe primary reflector dish.

FIG. 16 shows an example of an implementation of an offset Gregorianantenna arrangement in the example of a high gain subscriber module,showing the secondary reflector 42 and a planar array of antennaelements 41 arranged as a feed for transmitting radio frequency signalsto the secondary reflector 42, and/or for receiving radio frequencysignals from the secondary reflector 42. A conductive support block isconfigured to support the planar array of antenna elements 41. Thesupport block is formed as a first end of a feed support member 47, thefeed support member being directly connected, at an end opposite thefirst end, to a support body 48 configured to support the primaryreflector dish 43.

FIG. 17 a shows a typical profile, in a vertical cross-section throughthe offset Gregorian antenna arrangement, in a similar plane to that ofthe cross-section of FIG. 16 . The reflector surfaces are shown of theprimary reflector dish 43 and the secondary reflector 42. A practicalimplementation may comprise reduced sections of the theoretical curvesshown in FIGS. 17 a and 17 b . The offset Gregorian arrangement isarranged so that the secondary reflector and the array of antennaelements do not obscure the sector in which beams are intended to beformed from the primary reflector dish. Typically, the secondaryreflector is offset vertically, so that the azimuth sector is notobstructed. Typically, in a fixed wireless access system, a smallerangular range, over which beams are formed, is needed in elevation thanin azimuth. The planar array of antenna elements 41 is also shown. FIG.17 b shows a typical profile, in a horizontal cross-section through theoffset Gregorian antenna arrangement, again showing the reflectorsurfaces of the primary reflector dish 43 and the secondary reflector42, and the planar array of antenna elements 41. The primary reflectordish 43 has a parabolic shape in both the vertical and horizontalcross-sections. The secondary reflector dish 42 also has a parabolicshape in both the vertical and horizontal cross-sections.

FIG. 18 shows an oblique perspective view of a wireless station havingthe offset Gregorian antenna arrangement in an example, showing anaperture 50 for align the wireless station with a second wirelessstation by sight, the primary reflector dish 43, and a non-conductiveenclosure 49 enclosing the secondary reflector and its support.

FIG. 19 is a view of the offset Gregorian antenna arrangement from thedirection of a radiofrequency main beam which the offset Gregorianantenna arrangement is configured to form, in an example. It can be seenthat the primary reflector dish 43 is substantially rectangular in planview, viewed from a direction parallel to the direction of aradiofrequency main beam which the offset Gregorian antenna arrangementis configured to form. The primary reflector dish 43 may be formed ofpressed metal. This arrangement has been found to provide a compactdesign with high radiofrequency gain. The secondary reflector, which iscovered by the enclosure 49, may also have a substantially rectangularin plan view, viewed from a direction parallel to the direction of aradiofrequency main beam which the offset Gregorian antenna arrangementis configured to form.

In the case of the Gregorian antenna arrangement, the pre-determinedplurality of antenna weight vectors are configured to form a pluralityof beams from the primary reflector dish 43 as shown in FIG. 20 . FIG.20 shows that the position of each of the plurality of beams isoverlaid, in a similar arrangement to that of FIG. 8 . In the case ofFIG. 18 , a reduced set of beams is used for communication, shown withinan ellipse 35. At least some of the rows are truncated at each end. Theresulting truncated rows are longest towards the centre of the range ofelevation values.

Similarly as for the case of FIG. 8 without an offset Gregorian antennasystem, the orientations of the beams are arranged in a grid comprisinga plurality of rows 34, the beams of each row being spaced in angularposition in the row by a first angular separation 29 on a first axis 32,in this example azimuth. The angular positions of the beams of each rowis offset 30 on the first axis 32 by half of the first angularseparation 29 with respect to the angular positions of beams in anadjacent row. For example, it can be seen that each beam in row 34 isoffset by half the separation between beams 29 with respect to theangular positions of beams in an adjacent row.

The beam numbers, selected from a 1-120 range, shown in FIG. 20 relateto an optional the special case of an order of addressing each beam in asearch process, in which every fourth numerical beam is selected to formthe first sub-set of beams. The second subset of beams may be selectedby selecting the preceding 10 and following 10 beams in numerical orderof the numbering system. This selects a second subset of beams which atleast includes the six beams surrounding a selected beam. This providesa simple algorithm for selecting beams.

As shown in FIG. 20 . each row is separated from an adjacent row by thefirst angular separation 29 multiplied by cosine 30 degrees on a secondaxis, perpendicular to the first axis 32, so that each beam of theplurality of beams is arranged as an equilateral triangle with twoadjacent beams. In practice the positions of the beams will be subjectto errors due to the accuracy of the beamforming weights, taking intoaccount the effects of quantisation.

For an initial acquisition search, a first sub-set of the pre-determinedplurality of antenna weight vectors is selected, as shown as circledbeams 28. In this case 26 beams are selected for the sub-set.

FIG. 21 shows a plot of antenna gain for the beam arrangement of FIG. 17, showing the maximum gain that can be achieved at each point in thearrangement by selection of the best beam, similarly to FIG. 9 . Thecontours are in steps of 1 dB.

Similarly as for beams formed directly from an array of antenna elementsas in the case shown by FIG. 8 , in the acquisition process, asuccession of beams is formed in a first time sequence at the firststation using the first sub-set of the pre-determined plurality ofantenna weight vectors to send first messages.

Dependent on the receipt of a first message at the second station usinga first beam at the first station, and an acknowledgement from thesecond station that the first message has been received, the first beamis used as the basis of a finer refinement process. The acknowledgementfrom the second station may be carried in the first beam in the casethat the first beam is used for both transmit and receive in a timedivision duplex arrangement. A refined search is carried out using afurther succession of beams formed at the first station using a secondsub-set of the pre-determined plurality of antenna weight vectorsselected to form beams adjacent to the first beam. As shown in FIG. 17 ,the second sub-set is selected to form at least a ring of six beamssurrounding the respective first and second beams if the first or secondbeam is not at an edge of the grid. The second sub-set of beams is shownin FIG. 17 by the beams marked by squares, 39 a, 39 b, etc., whichsurround the first beam 38 which was selected in the initial acquisitionprocess. This provides an efficient process for selecting a best beamfor use after establishing initial communication.

FIG. 22 shows the arrangement of the feed beams generated by the arrayof antenna elements to feed the secondary reflector, and to produce thearrangement of beams from the primary reflector dish shown in FIG. 20and FIG. 21 . Each feed beam number in FIG. 22 produces thecorrespondingly numbered beam from the primary reflector dish as shownin FIG. 20 .

As can be seen, the relationship between the azimuth and elevationdirection of each feed beam and the azimuth and elevation direction ofthe respective beam from the primary reflector dish is a non-linearfunction of azimuth and elevation. As shown in FIG. 22 , thepre-determined plurality of antenna weight vectors is configured to formthe plurality of feed beams such that the orientations of the pluralityof beams is arranged in a distorted grid comprising a plurality ofcurved rows 36 a, 36 b, etc., each curved row providing a monotonicchange in azimuth 32 angle along the curved row, and a non-monotonicchange in elevation 33 angle along the curved row. As shown, each curvedrow 36 a, 36 b, etc., has an offset in elevation angle between thecentre of the curved row and either end of the curved row. In thisexample, the offset in elevation angle for a curved row is equal to theangular spacing in elevation between the curved row and an adjacentcurved row +/−50%. In this example, each curved row has a greaterelevation angle at the centre of the curved row than at either end ofthe curved row, and in this example each curved row has an approximatelyparabolic dependence of elevation angle on azimuth angle, within +/−50%of a true parabola.

An example of an equation which may be used to relate the azimuth andelevation direction of each feed beam (x1, y1) to the azimuth andelevation direction of the respective beam from the primary reflectordish (x, y) is as follows:x1=x ²((0.12−0.0052 y)y+0.17)+0.21 x ³ +x((0.16 y+0.55)y−13.)+y((−0.27y−0.55)y−0.27)−0.38y1=x ²((0.061−0.047 y)y−1.7)+0.077 x ³ +x((0.030y+0.00061)y−0.46)+y((0.45 y−0.48)y−14.)−5.8

The above equation has been found to be a useful approximation.

Both the first and second wireless station may form a plurality of beamsduring the process of establishing communication. In this case, thesecond station has an antenna comprising an array of antenna elementsand a beamforming network, the beamforming network being configured toform a beam using an antenna weight vector selected from apre-determined plurality of antenna weight vectors. The method comprisesproviding a second pre-determined plurality of antenna weight vectors atthe second station configured to form a second plurality of beams, theorientations of the second plurality of beams being arranged in a secondgrid comprising a plurality of rows, the beams of each row being spacedin angular position in the row by a second angular separation, theangular position of the beams of each row being offset by half of thesecond angular separation with respect to the angular positions of beamsin an adjacent row. A sub-set of the second pre-determined plurality ofantenna weight vectors are selected for use at the second station. Asuccession of beams are formed in a second time sequence at the secondstation using the sub-set of the second pre-determined plurality ofantenna weight vectors. Dependent on the receipt of a first message atthe second station using the first beam at the first station and asecond beam at the second station, forming the further succession ofbeams at the first station using the second sub-set of thepre-determined plurality of antenna weight vectors selected to formbeams adjacent to the first beam, and forming a further succession ofbeams at the second station using the second sub-set of the secondpre-determined plurality of antenna weight vectors selected to formbeams adjacent to the second beam.

FIG. 23 is a flow diagram of a method in an example, according to stepsS23.1, S23.2, S23.3 and S23.4.

In an example, the wireless communication system may have an operatingfrequency of is a least 50 GHz. In other examples, the wirelesscommunication system may have an operating frequency of greater than 28GHz, for example 28 GHz.

It is to be understood that any feature described in relation to any oneexample may be used alone, or in combination with other featuresdescribed, and may also be used in combination with one or more featuresof any other of the examples, or any combination of any other of theexamples. Furthermore, equivalents and modifications not described abovemay also be employed without departing from the scope of the invention,which is defined in the accompanying claims.

What is claimed is:
 1. A subscriber module of a fixed wireless access communication system, the subscriber module comprising: an offset Gregorian antenna arrangement comprising a primary reflector dish and a secondary reflector; an array of antenna elements arranged as a feed for the secondary reflector, the array of antenna elements and the secondary reflector being offset in a vertical axis with respect to a centre of the primary reflector dish; a beamforming network, the beamforming network being configured to form a beam using an antenna weight vector; and a processor configured to provide the antenna weight vector selected from a pre-determined plurality of antenna weight vectors to the beamforming network, wherein the processor is configured to provide the pre-determined plurality of antenna weight vectors configured to form a plurality of beams, the orientations of the plurality of beams being arranged in a grid comprising a plurality of rows, each of the pre-determined plurality of antenna weight vectors being configured to form a respective beam from the primary reflector dish of the Gregorian antenna arrangement by forming a respective feed beam from the array of antenna elements, wherein a relationship between the azimuth and elevation direction of each feed beam and the azimuth and elevation direction of the respective beam from the primary reflector dish is a non-linear function of azimuth and elevation.
 2. The subscriber module of claim 1, wherein the pre-determined plurality of antenna weight vectors is configured to form the plurality of feed beams such that the orientations of the plurality of beams is arranged in a distorted grid comprising a plurality of curved rows, each curved row providing a monotonic change in azimuth angle along the curved row, and a non-monotonic change in elevation angle along the curved row.
 3. The subscriber module of claim 2, wherein each curved row has an offset in elevation angle between the centre of the curved row and either end of the curved row.
 4. The subscriber module of claim 3, wherein said offset in elevation angle for a curved row is equal to the angular spacing in elevation, at the centre of the curved row, between the curved row and an adjacent curved row +/−50%.
 5. The subscriber module of claim 2, wherein each curved row has a greater elevation angle at the centre of the curved row than at either end of the curved row.
 6. The subscriber module of claim 2, wherein each curved row has an approximately parabolic dependence of elevation angle on azimuth angle, within +/−50% of a true parabola.
 7. The subscriber module of claim 1, wherein the array of antenna elements has 8 element columns and 8 element rows with a spacing between antenna elements in each element row and in each element column of substantially half a wavelength at an operating frequency of the wireless communication system.
 8. The subscriber module of claim 1, wherein each pre-determined antenna weight vector provides a respective phase shift for each antenna element.
 9. A method of forming a beam from a subscriber module of a fixed wireless access communication system, the subscriber module having an offset Gregorian antenna system comprising a primary reflector dish, a secondary reflector, an array of antenna elements and a beamforming network, the beamforming network being configured to form a beam using an antenna weight vector selected from a pre-determined plurality of antenna weight vectors, wherein the array of antenna elements is arranged to feed the secondary reflector to form the beam from the primary reflector dish, the method comprising: providing the pre-determined plurality of antenna weight vectors configured to form a plurality of beams, the orientations of the plurality of beams being arranged in a grid comprising a plurality of rows, each of the pre-determined plurality of antenna weight vectors being configured to form a respective beam from the primary reflector dish of the Gregorian antenna arrangement by forming a respective feed beam from the array of antenna elements, wherein the array of antenna elements and the secondary reflector are offset in a vertical axis with respect to a centre of the primary reflector dish, and a relationship between the azimuth and elevation direction of each feed beam and the azimuth and elevation direction of the respective beam from the primary reflector dish is a non-linear function of azimuth and elevation.
 10. The method of claim 9, wherein the pre-determined plurality of antenna weight vectors is configured to form the plurality of feed beams such that the orientations of the plurality of beams is arranged in a distorted grid comprising a plurality of curved rows, each curved row providing a monotonic change in azimuth angle along the curved row, and a non-monotonic change in elevation angle along the curved row.
 11. The method of claim 10, wherein each curved row has an offset in elevation angle between the centre of the curved row and either end of the curved row.
 12. The method of claim 11, wherein said offset in elevation angle for a curved row is equal to the angular spacing in elevation, at the centre of the curved row, between the curved row and an adjacent curved row +/−50%.
 13. The method of claim 10, wherein each curved row has a greater elevation angle at the centre of the curved row than at either end of the curved row.
 14. The method of claim 10, wherein each curved row has an approximately parabolic dependence of elevation angle on azimuth angle, within +/−50% of a true parabola.
 15. The method of claim 9, wherein the array of antenna elements has 8 element columns and 8 element rows with a spacing between antenna elements in each element row and in each element column of substantially half a wavelength at an operating frequency of the wireless communication system.
 16. The method of claim 9, wherein each pre-determined antenna weight vector provides a respective phase shift for each antenna element. 